The present invention relates generally to manufacturing nuclear reactor fuel assemblies and, more particularly, to an improved method and apparatus for enclosing a gas at a preselected pressure in the fuel rods which help make up a fuel assembly.
Conventional techniques for enclosing a pressurized inert gas in a nuclear fuel rod include sealing one end of the fuel rod, placing the open other end in a sealed chamber, evacuating or purging the sealed chamber, pressurizing the sealed chamber with inert gas to a predetermined pressure, and then sealing closed the open other end of the fuel rod while it is in the sealed chamber. Apparatus for welding closed the open end of a fuel rod while the rod is in a sealed chamber is disclosed in the U.S. Pat. Nos. 3,683,148 and 3,842,238 to Boyko et al. which are hereby incorporated by reference into this patent application. The enclosed pressurized gas helps prevent creep or collapse of the fuel rod, due to the pressure of the environment in which it is located during reactor operation, as is more fully explained in the U.S. Pat. No. 3,677,894, to Ferrari which is hereby incorporated by reference into this patent application.
With conventional gas enclosure techniques, a quantity of the fuel rod's residual atmosphere (including the moisture content therein) remains sealed within the fuel rod together with the introduced pressurized inert gas. This remaining residual atmosphere (including its moisture) can react with the interior of the fuel rod to degrade fuel performance, and even contribute to fuel rod cladding failure.